Thermoelectric generation is a technology for directly converting thermal energy into electric energy using the Seebeck effect, i.e. a phenomenon that when a temperature difference is given between opposite ends of a substance, thermoelectric power is generated in proportion to the temperature difference. This electric energy can be extracted as electric power, with a load being connected to form a closed circuit. This technology has been used practically, for example, for a remote area power supply, a space power supply, and a military power supply.
The thermoelectric cooling is a technology for causing heat absorption utilizing the Peltier effect, i.e. a phenomenon that when a current is passed through a circuit composed of different substances joined together, heat absorption occurs at one junction and heat generation occurs at the other junction. This effect is considered to be obtained due to the difference between the substances in ratio between a thermal flow and an electric current carried by electrons that flow when two types of substances with carriers whose types are different from each other, such as, for example, a p-type semiconductor and an n-type semiconductor, are connected to each other thermally in parallel and electrically in series. The technology of thermoelectric cooling has been used practically, for example, for local cooling devices for cooling electronic devices used in a space station or wine coolers.
Currently, there is a demand for a thermoelectric conversion material that exhibits good thermoelectric conversion characteristics (thermoelectric performance) over a wide temperature range from room temperature to a high temperature, and various materials, particularly semiconductors, are being studied.
Generally, the thermoelectric performance is evaluated by a figure of merit Z, or a figure of merit ZT that is obtained by multiplying Z by absolute temperature T to be non-dimensionalized. ZT can be expressed as ZT=S2/ρκ, where S is a Seebeck coefficient, ρ is electrical resistivity, and κ is thermal conductivity. That is, in order to obtain a thermoelectric conversion material with an excellent thermoelectric performance, a high thermoelectric power, low thermal conductivity, and low electrical resistivity are required. In the conventional thermoelectric conversion materials, however, it cannot always be said that a sufficiently high ZT is obtained. This is because S, ρ, and κ are basically functions of carrier density and therefore are difficult to vary independently, which results in difficulty in obtaining an optimal solution.
Examples of the thermoelectric conversion materials that have been developed so far include a Bi2Te3 semiconductor and with this material, a practical-level thermoelectric performance can be obtained at room temperature. Furthermore, materials with complicated structures, such as a skutterudite compound and a clathrate compound, also are being developed so as to be put into practical use.
JP 8(1996)-186294 A (Reference 1) discloses a thermoelectric conversion material represented by a formula Co1-xMxSb3 (in Reference 1, x is 0.001 to 0.2) in which a part of Co, which is an element constituting a CoSb3 compound having a skutterudite structure, is substituted with at least one element M selected from Pd, Rh, and Ru. However, the thermoelectric conversion material disclosed in Reference 1 has a problem in that it is oxidized in a high temperature range to have a deteriorated thermoelectric performance.
JP 9(1997)-321346 A (Reference 2) and JP 2005-64407 A (Reference 3) each disclose a thermoelectric conversion material, a so-called “AMO2 oxide” (where A is an alkali metal element or an alkaline-earth metal element, and M is Co in Reference 2 and Rh in Reference 3) and describe that such a cobalt oxide and a rhodium oxide exhibit an excellent thermoelectric performance. Furthermore, the thermoelectric conversion materials disclosed in References 2 and 3 each have a structure composed of electrical conducting layers formed of CoO2 or RhO2, and electrical insulating layers disposed between adjacent electrical conducting layers, i.e. a so-called layered bronze structure. This structure does not tend to be destroyed even under high temperature and does not undergo deterioration due to oxidation. Thus those materials are expected to be used in a high temperature range. However, the above-mentioned cobalt oxide and rhodium oxide have metallic properties, i.e. properties that the electrical resistivity increases with an increase in temperature, which may cause those materials to lack in thermoelectric performance in a high temperature range. Furthermore, for example, it is expected that thermoelectric generation in a higher temperature range as compared to the conventional case allows a higher electric energy to be generated. With the thermoelectric conversion material exhibiting metallic properties, however, the electrical resistivity increases with an increase in temperature, which results in an increase in loss.
Denis Pelloquin et al., “Partial substitution of rhodium for cobalt in the misfit [Pb0.7Co0.4Sr1.9O3]RS[CoO2]1.8oxide”, Journal of Solid State Chemistry, 178 (2005) pp. 769-775 (Reference 4) indicates that when in a cobalt oxide (Pb0.7Co0.4Sr1.9O3)(CoO2)1.8 with a structure including three electrical insulating layers disposed between electrical conducting layers, which is one of the layered bronze structures, a part of the Co atom is substituted by a Rh atom, semiconducting properties that the electrical resistivity decreases with an increase in temperature (for instance, a temperature derivative of electrical resistivity dρ/dT is negative) emerge. However, the electrical resistivity ρ of this material is considerably high, specifically at least 1 Ω·cm (see FIG. 5 of Reference 4), and therefore it cannot always be said to be a suitable thermoelectric conversion material. Moreover, this material contains Pb and therefore has a problem of considerably high burden on the environment.